The present invention relates to a drive unit with an electric motor.
The European patent document EP 0 175 996 A3 discloses a drive unit which has an electric motor with a motor shaft, a screw connected with the motor shaft, and a screw wheel engaging with the screw. As shown in FIG. 3 of this reference, both ends of the motor shaft are provided with friction brakes having a first friction element fixedly connected with the longitudinally displaceably supported motor shaft and the second friction elements secured from rotation in one rotary direction by a rotary direction stop. One rotary direction stop operates against left rotation and the other rotary direction stop operates against right rotation. A return drive moment which acts from the screw wheel in one or another rotary direction, for example of a moveable vehicle component, presses through the screw of the turned-off motor a first friction element for producing a frictional connection against the associated second friction element so that the motor shaft is braked. The friction brakes in this case have the disadvantage that the screw transmission composed of the screw and the screw wheel is prone to self locking, and this effect can lead to vibrations, and undesired, arbitrary displacement of the moveable vehicle components such as window panes, slidable roofs, mirrors, seats and the like. The above described friction brakes make difficult an unobjectionable opening of a window or a sliding roof. The components of the sliding roofs which are produced and designed with due regard to these phenomena are expensive.
U.S. Pat. No. 4,823,035 discloses an electric motor with an arresting mechanism which operates against undesirable rotation of the motor shaft and during turning on of the electric motor is disengageable by electric current. The arresting mechanism has a hub which is non rotatably connected with the motor shaft and has four radially projecting arresting teeth, an arresting pin for engaging in gaps between the arresting teeth which is supported radially displaceably relative to the hub and loaded by a spring in direction toward the hub, and an actuator acting for the disengagement and formed as a screw spring of wire. The wire is wound of a shape-memory material or metal and is heatable by an electric current to act, due to the heating, against the helical spring and to pull the arresting pin from a corresponding gap between the arresting teeth. The windings are connected in form of an electric coil with a switch on the one hand and with a brush or an armature winding of the electric motor in series on the other hand, so that the turning-on of the electric motor causes heating of the actuator and thereby finally a disengagement of the arresting mechanism. The turning off of the electric motor leads to a cooling of the actuator and finally to pressing the arresting pin by the helical spring towards the hub, so that the arresting of the pin between the arresting teeth is possible. A mechanical form-locking contact is thereby produced between the pin and at least one of the arresting teeth.